The remnants of the connector had a black covered wire and a blue covered
wire going to the cylinder parts of the connector. I don't suppose there is
a standard for these sorts of things , but it looks as though the blue one
was not connected through to the power supply so just one lead but connected
to the inside or the outside cylinder of the connection. This is a pin
centre conductor , not a female connector.
My guess is the inner surface as then both conductor surfaces are recessed
from casual touching anything when disconnected.